Also, it may sound stupid coming from my notably cheap mouth, but if she does decide she wants a nicer bike, don't just buy what you find a deal on. Take her shopping, involve her in the decision, don't put an artificially low price constraint on her. Help her if she requests it, but don;t dictate what she wants, let her make the decision. It is her butt on the bike, not yours, she has to want to be on it if you are trying to get her to centuries.
A great point that was made on another thread a few weeks ago: if she's not interested in making those decisions, she's not interested in doing what you are trying to get her to do.